backing-dev: bdi sb prune should be in the unregister path, not destroy

Commit 592b09a4 was different from
the tested path, in that it moved the bdi super_block prune from
unregister to destroy context. This doesn't fully fix the sync hang
bug on unexpected device removal, as need to prune the bdi cache
pointer before killing flusher thread.
Tested-by: NArtur Skawina <art.08.09@gmail.com>
Signed-off-by: NJens Axboe <jens.axboe@oracle.com>

mm/backing-dev.c

@@ -628,6 +628,8 @@ static void bdi_prune_sb(struct backing_dev_info *bdi)
 void bdi_unregister(struct backing_dev_info *bdi)
 {
 	if (bdi->dev) {
+		bdi_prune_sb(bdi);
+
 		if (!bdi_cap_flush_forker(bdi))
 			bdi_wb_shutdown(bdi);
 		bdi_debug_unregister(bdi);
@@ -697,7 +699,6 @@ void bdi_destroy(struct backing_dev_info *bdi)
 		spin_unlock(&inode_lock);
 	}
 
-	bdi_prune_sb(bdi);
 	bdi_unregister(bdi);
 
 	for (i = 0; i < NR_BDI_STAT_ITEMS; i++)